1. Field of the Invention
The present invention relates to a degradable high polymer network, that is, a hydrolyzable high polymer network. More particularly, the invention relates to a hydrolyzable high polymer network comprising a composition essentially consisting of a lactic acid-base polymer which is hydrolyzable in the natural environment.
2. Related Art of the Invention
Conventionally known plastic networks have been prepared by blending an organic or inorganic foaming agent with a general purpose resin based on polyolefin, polyvinyl chloride or polyamide in a specific proportion, conducting extrusion-foaming with a melt-extruding method, generating cells in the extrudate and stretching said extrudate to open said cells.
For example, Japanese Laid-Open Patent Sho 51-47170(1976) has proposed a nonwoven-fiber network obtained by conducting melt-foaming extrusion of a mixture of thermoplastic resin such as polyolefin, polyurethane and polyester with a foaming agent, and opening the extrudate.
The nonwoven-fiber network disclosed in said patent is useful for nonwoven fabrics and adhesive materials and also widely used for absorbing oils and body fluids, heat insulating materials, filter media and packaging materials. In these uses, particularly for absorbing oils and body fluids, filter media and packaging materials are generally so-called throw-away uses where these materials are abandoned immediately after use.
However, the nonwoven-fiber network composed of the above resin, that is, the high polymer network has a very slow rate of hydrolysis in the natural environment and remains semipermanently when buried under the ground. Disposal of the network in the ocean causes damage of aesthetics or destruction of the living environment of marine organisms. Thus, disposal of wastes has become a social problem with expansion of consumption.
So far, a high polymer network which can be hydrolyzed in the natural environment has not been known.
Polylactic acid and its copolymer has been known as a thermoplastic and hydrolyzable polymer. These lactic acid-based polymers can be obtained by fermentation of inexpensive materials such as corn starch and corn syrup and can also be prepared from petrochemicals such as ethylene.
U.S. Pat. No. 1,995,970 discloses a preparation process of the lactic acid-base polymer by polymerization of lactic acid, lactide or a mixture of these compounds. The lactic acid-base polymer has been used for surgical sutures and medical slow release matrixes because of good biological compatibility and hydrolyzability. The lactic acid-based polymer, however, is generally high in hardness and hence has a disadvantage of lacking in flexibility when used in the form of a film. Consequently, the lactic acid-based polymer has many restrictions in use.
A high polymer network of a polylactic acid-base resin which is plasticized to provide flexibility, if desired, and can be used for hydrolyzable absorbing materials, heat-insulation materials, filter media and packaging materials, has not yet been known.